1. Field of the Invention
The present invention relates to a calibration method and an endoscope system, using semiconductor light sources such as R-, G-, and B-LEDs.
2. Description Related to the Prior Art
Endoscope systems have been widely used in the medical field. The endoscope system comprises a light source device, an endoscope, and a processor device. Endoscopic observation with the endoscope system includes distant-view observation and close-up observation of an observation object. The observation from a distant view is referred to as the distant-view observation. The observation in close proximity to the observation object is referred to as the close-up observation. The observation distance from the observation object is changed depending on the purpose of the observation or diagnosis. A change in the distance from the observation object significantly changes brightness of the observation object. For this reason, it is desirable for the endoscope system to sufficiently ensure a dynamic range (resolution) of illumination light that is used for illuminating the observation object. Since scopes with various levels of sensitivity and color reproducibility are connectable to the endoscope system, ensuring the sufficient dynamic range of the illumination light is also desirable for covering variations between the scopes.
As described in Japanese Pat. No. 4694285, the dynamic range of the illumination light suitable for a scope type is ensured by changing the maximum aperture ratio of an optical diaphragm in accordance with the scope type. However, it is difficult to finely adjust the light amount of the illumination light, namely, to ensure the sufficient dimming resolution, with the use of the optical diaphragm because the optical diaphragm mechanically adjusts the light amount of the illumination light.
Instead of controlling the light amount with the optical diaphragm, a semiconductor light source such as an LED (light emitting diode) may be used as an illumination light source, to widen the dynamic range of the illumination light and to improve the minimum resolution. Thereby, both the light intensity and the emission time of the illumination light are controlled. However, a pulse signal for driving the semiconductor light source is not a perfect rectangular wave, but has a nonlinear shape due to pulse rise delay (overshoot) or pulse fall delay (undershoot). The pulse signal becomes stable in a very short period of time, so that the light which corresponds to the nonlinear portion of the pulse signal due to the pulse rise delay or the like does not cause a problem in a case where the light amount (dose) of the illumination light is high or the emission time of the illumination light is long. However, a ratio of the light which corresponds to the nonlinear portion of the pulse signal increases in a case where the dose of the illumination light is extremely small. As a result, the actual dose may differ from the specified dose (target dose). This causes problems, for example, image quality degradation due to a shortage of brightness or color changes.
It is known that wavelengths of light from the LED tend to shift when intensity of the light increases. The wavelength variation (fluctuation) of the LED affects linearity characteristics of the illumination light, resulting in image quality degradation. In a case of narrowband light observation in which narrowband light of specific wavelengths is used, the wavelength variation may change an image of blood vessels and the like. This may also degrades the image quality.
In a case where a semiconductor light source such as the LED is used as the illumination light source as described above, it is desirable to perform high-resolution light amount control in the endoscope system to capture a high-quality image under light of necessary brightness (intensity) and display it while influences of the pulse rise delay, the wavelength variations, and the like are suppressed.
In a case where the illumination light is generated by the combined use of the LEDs of different colors such as an R-LED, a G-LED, and B-LED, instead of the use of the LED of a single color, emission spectra of the light from the respective LEDs are not flat (uniform). Due to the wavelength variations of the LEDs and variations in spectral sensitivities of the image sensors, the combined use of the LEDs of different colors may cause the image quality degradation more frequently than the use of the single color LED. In a case where the LEDs differ from each other in the dimming resolution, the light amounts of the light of the respective colors may vary and cause color changes, depending on the observation mode or the scope type. Such color change problems become more pronounced as the light amount of the illumination becomes extremely small. To reduce the color changes, the color balance may be adjusted by correcting the white balance in accordance with the observation mode or the scope type. However, the white balance correction is not preferred because the white balance correction uses an analog gain that amplifies electric noise. Therefore it is desirable to eliminate the color changes and the like through adjustment of the light intensity ratio among the LEDs while the analog gain is maintained at a low level, to the extent that the noise is not noticeable.